Chinese satellite images have detected a large object in the southern Indian Ocean that could be part of missing Malaysia Airlines flight MH370.

The latest possible lead came as the search for the missing plane entered its third week, with still no confirmed trace found of the Boeing 777 or the 239 people on board.

Australian authorities have plotted the latest object spotted by China ahead of a fourth day of searching for the jet in the search zone about 2,500 kilometres south-west of Perth.

The new potential sighting was dramatically announced by Malaysia's acting transport minister, Hishammuddin Hussein, after he was handed a note with details during a news conference in Kuala Lumpur, scooping the official announcement from China.

"Chinese ships have been dispatched to the area," Mr Hishammuddin said.

China said the object was 22 metres long and 13 metres wide, and was spotted about 120 kilometres south-west of two possible objects seen on satellite images taken on March 16 and announced by the Australian Government on Thursday.

The image was captured by the high-definition Earth observation satellite Gaofen-1 early on March 18, two days after the Australian satellite picture was taken, China's State Administration of Science, Technology and Industry for National Defence (SASTIND) said on its website.

On Saturday, the search crews had taken advantage of good weather, with 10 kilometres of visibility, according to officials - a crucial boost for a search that is relying more on human eyes than the technical wizardry of the most advanced aircraft in the world.

No sign of plane in northern corridor but search will continue

Where the missing plane went after it flew out of range of Malaysia's military radar off the country's north-west coast has been one of the most puzzling aspects of what has quickly become perhaps the biggest mystery in modern aviation history.

Electronic "pings" detected by a commercial satellite suggested it flew for another six hours or so, but could do no better than place its final signal on one of two vast arcs: a northern corridor from Laos to the Caspian Sea, and a southern one stretching from Indonesia down to the part of the Indian Ocean that has become the focal point of the search.

Malaysia has said the search will continue in both corridors until confirmed debris is found.

China, India, Pakistan, Myanmar, Laos, Kyrgyzstan and Kazakhstan had all said, based on preliminary analysis, that there have been no sightings of the aircraft on their radar, Mr Hishammuddin said.

Tracking flight MH370

Aircraft and ships have renewed the search in the Andaman Sea between India and Thailand, going over areas in the northern corridor that have already been exhaustively swept.

The Pentagon said it was considering a request from Malaysia for sonar equipment.

The P8 and P3 spy planes, which the United States is already deploying in the search, also carry "sonobuoys" that are dropped into the sea and use sonar signals to search the waters below.

The search itself has strained ties between China and Malaysia, with Beijing repeatedly leaning on Kuala Lumpur to step up its hunt and do a better job at looking after the relatives of the Chinese passengers, who made up two-thirds of the flight.

Flight MH370 vanished from civilian radar screens early into its flight on March 8, less than an hour after taking off from Kuala Lumpur on a scheduled flight to Beijing.

Investigators believe someone on board shut off the plane's communications systems, and partial military radar tracking showed it turning west and re-crossing the Malay Peninsula, apparently under the control of a skilled pilot.

That has led them to focus on hijacking or sabotage, but they have not ruled out technical problems.

For families of the passengers, the process has proved to be an emotionally wrenching battle to elicit information.

In a statement on Saturday, relatives in Beijing lambasted a Malaysian delegation for "concealing the truth" and "making fools" out of the families after they said they left a meeting without answering all their questions.

"This kind of conduct neglects the lives of all the passengers, shows contempt for all their families, and even more, tramples on the dignity of Chinese people and the Chinese government," they said.

Some experts have argued that the reluctance to share sensitive radar data and capabilities in a region fraught with suspicion amid China's military rise and territorial disputes may have hampered the search.